Heretofore, as an automatic open/close controlling valve for gas appliances such as city gas appliances, there has been used a small-sized solenoid valve which is operated using a dry cell or the like. Calculation will now be made about the force required for opening and closing a 28 mm dia. in a city gas pipe for example used at an end user. For example, in a solenoid valve wherein fluid of gas pressure of 500 mm in terms of water column is opened and with a 28 mm dia. valve, a back pressure of about 300 g is exerted on the back of the valve in a closed state. An electromagnet for opening and closing such a valve requires a valve holding spring force of 250 g, so it has heretofore been necessary to use a large-sized electromagnet having an initial pull-up force of 550 g and a stroke of 6 mm or so.
Thus, in the prior art, a large-sized electromagnet is required because of a great influence of the valve back pressure, so it is impossible to adopt a power source of a very small capacity such as a dry cell for example and the use of a commercial power source is unavoidable, thus resulting in increase of the equipment cost.
There has been proposed a technique of attaching a small pilot valve for pressure equalizing to a valve, opening pilot valve prior to opening of the valve to eliminate the difference between upstream pressure and downstream pressure, thereby decreasing the force required for valve opening and closing to reduce the differential force of the valve. According to such technique, there have been known a construction wherein a pilot valve is disposed in a by-pass, and a construction wherein a valve element itself is formed with a pilot valve to provide a two-stage motion structure for a valve stem of a valve body between pilot valve open and valve body open.
Such conventional technique of providing a by-pass and mounting a separate pilot valve therein and technique of incorporating a separate pilot valve in a valve body are disadvantageous in that both are complicated in structure and expensive.
There also is a problem such that the lower limit value of electric power consumed for positively a solenoid valve using such valve varies greatly in each operation. For ensuring opening/closing operation even at a maximum variation, it is necessary to use a battery of a high voltage for example and set the power consumption rather high in each operation, counting a considerable safety factor. A further reduction of the power consumption has been desired. In an ordinary valve for gas, it is required that there be no leakage of fluid when the urging force of a spring for pressing a valve body against a valve seat is 80 g and the back pressure is 1,000 mm to 20 mmH.sub.2 O. In such a conventional valve as illustrated in FIG. 9, however, the back pressure is not always constant. In a solenoid valve for opening and closing a valve of D=28 mm, when the back pressure is 20 mmH.sub.2 O for example, it is almost the urging force of the spring alone that serves as a valve closing force, so there has been a likelihood of increased leakage of fluid between a valve seat 2 and a valve body 1. In this conventional example, leakage occurs at a back pressure below 150 mmH.sub.2 O. The numeral 3 denotes a valve stem and numeral 6 denotes a presser member for the valve body 1.
It is the object of the present invention to solve the above-mentioned problems of the prior art and provide an improved valve capable of being operated and closed with a very small force or a very small power source such as a battery without the need of using such a separate valve as has been referred to above, and also provide an improved technique relating to a solenoid valve using such improved valve.